Leaf litter breakdown in streams of East Malaysia, Borneo: a study of biodiversity and ecosystem functioning

Litter decomposition in streams represents an important terrestrial-aquatic link in tropical forests and studies have shown that detritivore shredders are more abundant in temperate than tropical streams because the feeding guild is believed to be more adapted to cool waters where there is greater availability of high quality resources. I hypothesized that this global variation in shredder distribution may be observed at local scales in tropical regions along an altitudinal gradient and investigated the importance of shredders in litter breakdown using field-based techniques in streams flowing through the different forest types of East Malaysia (Borneo) that correspond to different altitudes (21 - 3393 m above sea level). I found evidence of increasing shredder abundance and relative abundance with altitude as a possible function of decreasing temperature and decreasing lignin concentrations in litter. The altitudinal variations in taxonomic composition of shredder assemblages were distinct between lowland streams that were dominated by semi-aquatic cockroach, beetle (Elmidae) and stonefly (Neumouridae and Cryptoperla) shredders, and highland streams that were represented by Cranefly (Tipulidae) and Caddisfly (Lepidostomatidae) shredders. Environmental effects on shredder distribution were detected, but this did not translate into a parallel functional trend of increased shredder-mediated decomposition of local riparian litter with increasing elevation. Initial nitrogen content of litter, which was not altitude related, was a strong and consistent predictor of decomposition rates throughout the study. Diversity of shredders in lowland streams ranged from 0 to 8, and species composition was highly variable. Conversely, in highland streams, diversity was more stable in terms of both species richness (range 3 to 5) and composition. The high variability, across lowland streams, has typically been observed in other tropical surveys of shredder distribution. We may have captured the effect of this variable distribution on decomposition, since biodiversity effects on decomposition were detected in our study. Whether this relationship is a direct function of species interactions through complementary or facilitative effects, or strongly mediated by the presence of large shredders typical to lowland streams, remains untested. The relationship between shredder communities and environmental conditions may be general, transcending local factors, because this association is observed at both latitudinal and altitudinal gradients. In contrast, ecosystem functioning may not follow this general trend since biodiversity effects on decomposition are highly specific, reliant on shredder species identity, functional traits, composition and interactions. Shredder community structure may show patterns across broad biological scales, but functional patterns would be reliant on the local interactions within these structures

Leaf litter breakdown in streams of East Malaysia, Borneo: a study of biodiversity and ecosystem functioning

Litter decomposition in streams represents an important terrestrial-aquatic link in tropical forests and studies have shown that detritivore shredders are more abundant in temperate than tropical streams because the feeding guild is believed to be more adapted to cool waters where there is greater availability of high quality resources. I hypothesized that this global variation in shredder distribution may be observed at local scales in tropical regions along an altitudinal gradient and investigated the importance of shredders in litter breakdown using field-based techniques in streams flowing through the different forest types of East Malaysia (Borneo) that correspond to different altitudes (21 - 3393 m above sea level). I found evidence of increasing shredder abundance and relative abundance with altitude as a possible function of decreasing temperature and decreasing lignin concentrations in litter. The altitudinal variations in taxonomic composition of shredder assemblages were distinct between lowland streams that were dominated by semi-aquatic cockroach, beetle (Elmidae) and stonefly (Neumouridae and Cryptoperla) shredders, and highland streams that were represented by Cranefly (Tipulidae) and Caddisfly (Lepidostomatidae) shredders. Environmental effects on shredder distribution were detected, but this did not translate into a parallel functional trend of increased shredder-mediated decomposition of local riparian litter with increasing elevation. Initial nitrogen content of litter, which was not altitude related, was a strong and consistent predictor of decomposition rates throughout the study. Diversity of shredders in lowland streams ranged from 0 to 8, and species composition was highly variable. Conversely, in highland streams, diversity was more stable in terms of both species richness (range 3 to 5) and composition. The high variability, across lowland streams, has typically been observed in other tropical surveys of shredder distribution. We may have captured the effect of this variable distribution on decomposition, since biodiversity effects on decomposition were detected in our study. Whether this relationship is a direct function of species interactions through complementary or facilitative effects, or strongly mediated by the presence of large shredders typical to lowland streams, remains untested. The relationship between shredder communities and environmental conditions may be general, transcending local factors, because this association is observed at both latitudinal and altitudinal gradients. In contrast, ecosystem functioning may not follow this general trend since biodiversity effects on decomposition are highly specific, reliant on shredder species identity, functional traits, composition and interactions. Shredder community structure may show patterns across broad biological scales, but functional patterns would be reliant on the local interactions within these structures

Stream ecosystem integrity is impaired by logging and shifting agriculture in a global megadiversity center (Sarawak, Borneo)

In common with most of Borneo, the Bakun region of Sarawak is currently subject to heavy deforestation mainly due to logging and, to a lesser extent, traditional slash-and-bum farming practices. This has the potential to affect stream ecosystems, which are integrators of environmental change in the surrounding terrestrial landscape. This study evaluated the effects of both types of deforestation by using functional and structural indicators (leaf litter decomposition rates and associated detritivores or 'shredders', respectively) to compare a fundamental ecosystem process, leaf litter decomposition, within logged, farmed and pristine streams. Slash-and-bum agricultural practices increased the overall rate of decomposition despite a decrease in shredder species richness (but not shredder abundance) due to increased microbial decomposition. In contrast, decomposition by microbes and invertebrates was slowed down in the logged streams, where shredders were less abundant and less species rich. This study suggests that shredder communities are less affected by traditional agricultural farming practices, while modem mechanized deforestation has an adverse effect on both shredder communities and leaf breakdown

Leaf-litter breakdown in tropical streams: is variability the norm?

Many forested headwater streams are heterotrophic ecosystems in which allochthonous inputs of plant litter are a major source of energy. Leaves of riparian vegetation entering the stream are broken down by a combination of biotic and abiotic processes and, in most temperate and boreal streams, provide food and habitat for dense populations of detritivorous invertebrates. However, tropical streams in different parts of the world show substantial variability in the number and diversity of leaf-shredding detritivores (hereafter detritivores). We used data obtained with standardized methods from multiple streams in Africa, the Americas, Asia, and Australia to test the hypothesis that this variability would lead to differences in the relative role of detritivores and microorganisms in the breakdown process.We also tested the hypotheses that variability in litter breakdown rates changes with litter type (native litter mixtures vs nonnative alder [Alnus glutinosa]) and is higher across regions within than outside the tropics. We found that litter breakdown rates were highly variable across sites, with no consistent pattern within geographic areas, although litter consumption by detritivores was negligible at several sites, all in America. Geographic patterns of litter breakdown also varied between litter types, with higher breakdown rates for alder than for native litter in most but not all regions.When litter breakdown rates at the tropical sites were compared to previously reported values from temperate and boreal regions, we found that differences in variability between tropical and temperate sites were inconsistent, with great differences among studies. Further global-scale studies will be needed to assess the extent to which latitudinal changes in the diversity and composition of microbial and detritivore assemblages contribute to variability in litter breakdown rates

Data from: Biotic and abiotic variables influencing plant litter breakdown in streams: a global study

Plant litter breakdown is a key ecological process in terrestrial and freshwater ecosystems. Streams and rivers, in particular, have high rates of carbon dioxide evasion and they contribute substantially to global carbon fluxes. However, there is little information available on the relative roles of different drivers of plant litter breakdown in fresh waters, particularly at large scales. We present a global-scale study of litter breakdown in streams to compare the roles of biotic, climatic and other environmental factors on breakdown rates. We conducted an experiment in 24 streams encompassing latitudes from 47.8ºN to 42.8ºS, using litter mixtures of local species differing in quality and phylogenetic diversity, and alder (Alnus glutinosa) to control for variation in litter traits. Our models revealed that breakdown of alder was driven by climate, with some influence of pH, while variation in breakdown of litter mixtures was explained mainly by litter quality and phylogenetic diversity. The influence of these litter variables and pH was modulated by temperature, indicating that different mechanisms may operate at different latitudes. These results reflect global variability caused by multiple factors, but unexplained variance points to the need for expanded global-scale comparisons

A global experiment suggests climate warming will not accelerate litter decomposition in streams but might reduce carbon sequestration

The decomposition of plant litter is one of the most important ecosystem processes in the biosphere and is particularly sensitive to climate warming. Aquatic ecosystems are well suited to studying warming effects on decomposition because the otherwise confounding influence of moisture is constant. By using a latitudinal temperature gradient in an unprecedented global experiment in streams, we found that climate warming will likely hasten microbial litter decomposition and produce an equivalent decline in detritivore-mediated decomposition rates. As a result, overall decomposition rates should remain unchanged. Nevertheless, the process would be profoundly altered, because the shift in importance from detritivores to microbes in warm climates would likely increase CO₂production and decrease the generation and sequestration of recalcitrant organic particles. In view of recent estimates showing that inland waters are a significant component of the global carbon cycle, this implies consequences for global biogeochemistry and a possible positive climate feedback